The problem here is that manufacturers can not possibly know how you will use the spinner. All the clairvoyant engineers were spirited away by Microsoft back in the late 70's Therefore, they make the holes small so you, the end user, can enlarge them to suit your specific application. As DarZeelon hinted, it's always easier to remove material than it is to put it back. If the companies made the holes big enough to suit the largest propeller, then the holes would be much too large for many more common applications, and then those people would complain. It's a no-win situation for the manufacturers, so they go with the least-common denominator.

FWIW, it's really hard to get an aluminum spinner much out of balance, unless you're really ham-fisted about enlarging the holes. I generally cover the spinner with masking tape and map out my cuts, then carefully whittle away with a Dremel, and the spinners come out balanced just as well as they did from the factory.

Tru-Turn is the only spinner manufacturer out there that will custom-cut the holes in your spinner for the specific prop you want to use.